The term multi-wavelength detector as applied to the art of liquid chromatography and as used throughout this specification refers to an absorbance detector that is capable of "simultaneously" detecting the absorbance characteristics of a sample solution at more than one wavelength. What is meant by the term "simultaneously" is that a series of measurements at different wavelengths are made in a sufficiently short period of time so that only slight concentration changes occur in the sample being analyzed during the period of measurement. This terminology (i.e., multi-wavelength) is to be contrasted with the terms "variable" or "tunable" wavelength detectors which imply an off-line adjustment made to the absorbance detector so that different sample runs can be analyzed at different wavelengths in the range of interest.
In the past, ultraviolet/visible absorbance detectors have been very popular for use with HPLC systems because a significant number of compounds absorb radiation in the ultraviolet/visible range. The majority of these devices operate by analyzing a given sample solution at a single wavelength whether through use of a line emission source in combination with wavelength selective filters or a wide-range continuous source in combination with a monochromator. Problems arise with this measurement technique if some of the compounds being analyzed are only slightly absorbing at the wavelength at which the sample is irradiated. This produces an unacceptably low signal to noise ratio for those compounds.
It should be noted that where absorbance detectors are commonly said to operate at a specific wavelength, in all cases where the source emits a continuum of radiation as opposed to line emissions at fixed wavelengths, the detector is actually operating over a wavelength interval, the extent of which is determined by the spectral bandwidth of the instrument.
In order to improve upon the response of such single wavelength absorbance detectors, and to provide additional information with respect to the compound of interest, it has been proposed to measure absorbance of the sample at different wavelengths, and even to form a ratio of absorbances at different wavelengths, within time intervals short enough to minimize concentration changes in the sample. An example of such a multi-wavelength device is described in an article by Koichi Saltoh and Nobuo Suzuki that appeared in Analytical Chemistry, Vol. 51, Number 11, September 1979. The spectrophotometer described there is a dual beam optical instrument capable of scanning wavelengths between 200 and 800 nanometers (nm) which employs two alternate, continuous light sources--a 30 watt deuterium and a 30 watt Tungsten lamp. The wavelength of the monochromator depends on the angular position of a diffraction grating, which is controlled by an electrically driven motor/clutch arrangement. The grating is moved across the full range of the desired spectrum at a rate of either two or one Hz. A shaft encoder senses the angular position of the grating. During each scanning cycle, absorbance data at the desired wavelength is extracted by the photodetector system which includes photo multiplier tubes for appropriate amplification. The short duration during which absorbance is measured at any given wavelength results in only a very limited signal level to be recorded at the photodetectors due to the low radiated power of the lamp. This causes signal to noise ratios to be low, limiting the sensitivity of the measurement.
Another attempt to utilize multi-wavelength absorption detection is found in the Model 165 Multichannel Rapid Scanning UV-Vis Detector manufactured by Beckman Instruments. This device also is a dual beam instrument that uses a conventional deuterium lamp that is continuously energized when the instrument is operated to generate radiation over the wavelength range of interest, but rather than provide a constantly moving diffraction grating, a positioning system is used to rapidly move the grating to an assigned position corresponding to the desired wavelength. This grating position is then maintained for a sufficient period of time to receive enough radiant energy at the photodectors to obtain a usable signal level. However, the low radiant energy output of the deuterium source requires this "holding period" to be sufficiently long to severly limit the number of wavelengths at which absorbance can be measured. Furthermore, due to the short sample measurement duration available if successive measurements are to appear to be simultaneous, it is necessary that the grating be rapidly displaced to the correct position for the next wavelength at which the sample is to be monitored. Thus it is apparent this system involves overcoming a significant amount of inertia in constantly starting, driving and stopping the grating, all of which must be achieved at relatively high speeds. Such a system requires low friction mechanical components and suffers from further difficulties associated with stopping the high speed movement of the grating precisely at the required position without undergoing undesirable oscillations. Moreover, the Beckman system relies upon a closed loop servo positioner and associated sensor to accurately position the grating, all of which adds to the complexity of the detector.
The foregoing examples of prior art multi-wavelength absorbance detectors exhibit certain fundamental operative limitations by using a continuous source in the measurement of more than one wavelength. This results in a tradeoff between the time involved in making the measurement and the amount of radiant energy received at the detector. If the measurement period is made long enough to allow sufficient energy to pass at the wavelength in question and thus increase the signal to noise ratio, either the number of wavelengths that can be measured simultaneously will be limited or fast eluting peaks will not be detected. On the other hand, short measurement period systems have low signal to noise ratios. While high-intensity pulsed sources that operate over a wide wavelength continuum have been proposed in the past (see for example U.S. Pat. No. 3,810,696), such uses have been in single wavelength applications. Moreover an important consideration in this prior patent is to overcome the undesirable heating effects of high power, continuous radiation sources while increasing source intensity.